This invention relates to a method of bonding dissimilar metals and the resulting structure therefrom and more particularly to an improved method for forming a valve seat for a reciprocating machine such as an engine and the resulting valve seat.
In many instances, it is the practice to form a component of a mechanism from dissimilar metals. This technique is resorted to so as to take the advantages of the individual properties of the two dissimilar materials in their specific locations. For example, it may be desirable to form components of a mechanism such as an engine from a lightweight material such as aluminum or an aluminum alloy. However, this type of material has certain wear disadvantages. Therefore, in areas of high wear, it is a practice to utilize a material that is somehow fixed to the base aluminum material, but which has more wear resistant characteristics.
A specific example of such an arrangement is the cylinder head of an engine. In the cylinder head, an aluminum alloy is frequently used as the base casting material because of the higher heat conductivity and lighter weight of the base aluminum alloy. However, in areas where wear is likely to incur, such as the valve seats, a harder, more wear resistant material is employed. Frequently, sintered iron or other similar materials are utilized to form the valve seats of the engine.
This gives rise to a problem in adhering the sintered iron valve seat to the aluminum alloy body of the cylinder head. Frequently, a press-fit arrangement is employed for forming such joints and FIG. 1 illustrates a section through a cylinder head having a press fit valve seat utilizing these two materials. The disadvantages of this type of construction will be described by particular reference to this figure after the basic structure of the cylinder head is described in conjunction with the figure.
In FIG. 1, a cylinder head assembly is indicated generally by the reference numeral 21. Only a portion of the cylinder head assembly 21 is illustrated because the invention deals, as aforenoted, primarily with the connecting arrangement between the valve seat inserts and the base cylinder head material. Therefore, a cross-sectional view taken through a portion of the cylinder head that forms a portion of the combustion chamber will permit those skilled in the art to understand the problems associated with this basic type of prior art construction.
The cylinder head 21 has a lower sealing surface 22, that is adapted to be brought into sealing engagement with a cylinder block which is not shown so as to form a closure for the cylinder bores at one end thereof. The surface 22 is interrupted by recesses 23 which cooperate with the cylinder bores and associated pistons to form the combustion chambers of the engine.
One or more exhaust ports 24 extend through one side of the base cylinder head material 25 and terminate in valve seats, indicated generally by the reference numeral 26. Each valve seat 26 is comprised of a generally cylindrical machined recess 27 formed in the cylinder head material 25 and into which is pressed a valve seat insert 28. Typically, the material of the cylinder head casting 25 will be aluminum or an aluminum alloy, while the valve seat insert 28 will be formed from a material that is harder and more wear-resistant, such as a sintered iron or the like.
A valve 29 is supported in a valve guide 31 in the cylinder head and is operated by a suitable actuating mechanism of any known type for opening and closing the valve seat 26 in a manner well known in the art.
A further intake passage 32 is formed also in the base cylinder head material 25 and has a valve seat insert 33 formed therein. The valve seat insert 33 is assembled in the same manner as the exhaust valve seat and, therefore, further description of it is not required.
An intake valve 34 is supported in a valve guide 35 in the cylinder head material 25, and is also operated by a suitable valve actuating mechanism.
It should be readily apparent that it is extremely important that the valve seat inserts 26 and 33 must be firmly attached in the cylinder head base material. There is a number of reasons for this. First, it must be ensured that the inserts do not fall out of the cylinder head when the engine is running. This would obviously cause serious damage to the engine.
Therefore, a technique using a force fit has been employed for placing the inserts into the cylinder head. This is accomplished by forming the bores in which the insert rings are placed with a smaller diameter than the outer diameter of the insert ring. The cylinder head is then heated and/or the insert ring is chilled so that the thermal expansion will permit the insert to be forced into the bore of the smaller diameter.
This requires considerable force, and when the materials return to their original temperatures, there will be a compressive force on the insert and a tensile force on the cylinder head, which can result in cracking of one or both of these components.
Furthermore, because of the difference of thermal expansions between the cylinder head material and the insert ring, the force fit must be such that when the engine becomes heated, the higher thermal expansion of the base cylinder head material will not cause the insert ring to become loosened, and fall out of its position. This requires an initial stress that is quite high.
In order to resist this cracking, it is necessary to provide an insert ring that has substantial thickness and also substantial length. This means that the heat transfer between the valves and the base cylinder head material is retarded by the interpositioning of this less thermally conductive material.
Furthermore, because of the sizes required, the configuration of the ports are also dictated in large part by the dimensions of the insert ring.
In addition to all of these disadvantages, the distance between adjacent valve seats must be kept relatively large so as to provide sufficient base cylinder head material between the valve seats to withstand the stresses required by the press fitting. Thus, it is not possible to utilize as large a valve area in the engine as might be desired.
In order to overcome these difficulties, it has been proposed to form some method of more permanently adhering the insert ring material into the base cylinder head material. Such techniques as welding or laser cladding have been proposed. Generally, these techniques require the application of sufficient heat so as to cause a melting to occur between the outer surface of the insert ring material and the inner surface of the cylinder head material. This forms a resulting alloy in the interface that is comprised of a mixture of the materials of the basic cylinder head and of the insert ring.
Although these welding or laser cladding techniques provide a more permanent bond, they do not totally solve the aforenoted problem. For example, because of the melting which occurs, there may be formed gas pockets or cavities in the interface between the two materials. These obviously will reduce not only the strength of the resulting bond, but will also reduce the heat transfer from the valve to the basic cylinder head material.
In addition, the resulting composition of the alloy at the interface can provide both metallurgical and physical properties that are not desirable. The alloy may, in fact, impede the heat transfer, or can reduce the strength or wear resistance of the resulting structure.
Therefore, there have been proposed techniques for forming a metallurgical bond between the insert ring and the cylinder head. The insert ring is pressed into place, and the insert ring and cylinder head material is heated sufficiently so as to form a metallurgical bond between the insert ring and the cylinder head at the interface, without any significant alloying of the two materials. This is also done in such a way that there is not a significant degree of liquification of the materials of either the insert ring or the cylinder head material that would cause voids. The following applications for United States Letters Patent, all of which are assigned to the Assignee hereof, and many of which are invented by the inventors hereof, are examples of methodology and valve seat insert formations that have been proposed so as to solve these problems and which, in fact, do solve these problems:
______________________________________ Ser. No. Filing Date ______________________________________ 08/278,026 July 20, 1994 08/483,246 June 7, 1995 08/601,287 February 16, 1996 08/607,823 February 27, 1996 ______________________________________
and others.
It is an object of this invention to provide an improved method of forming a metallurgical bond between two dissimilar metals that will overcome the problems of the prior art and which will offer a further improvement in the resulting strength of the bond from those noted copending applications.
As has been noted, this type of technique is utilized in environments where the resulting structure may be exposed to considerable thermal stresses. It has been found that although high initial bonding strength may result from certain techniques, when the resulting bond has been subjected to high temperatures for a length of time, the bonding strength may deteriorate.
It is, therefore, a further object of this invention to provide an improved bonding method wherein the bonding technique is such that the joint will maintain high strength, even after prolonged periods of operation at high temperatures.